Sinusoidal Alternating Current Ballast For Fluorescent Emergency Lighting

ABSTRACT

A ballast for a lamp has a battery that provides direct current in response to a loss of power. The ballast has a conversion circuit that is configured to receive the direct current from the battery and convert it into alternating current that is provided to a lamp. The alternating current is substantially sinusoidal, and has a current crest factor of 1.7 or less. The conversion circuit can use a push-pull topology. The ballast can also include a cathode-heating circuit for pre-heating a cathode associated with the lamp. The cathode-heating circuit pre-heats a cathode of the lamp by increasing the current output of the conversion circuit. The ballast can also include circuitry for converting mains power to power appropriate to illuminate the lamp.

CROSS REFERENCE TO RELATED APPLICATIONS

This non-provisional patent application claims priority to U.S.Provisional Patent Application No. 61/234,987, filed on Aug. 18, 2009,the entire contents of which are hereby fully incorporated herein byreference. This application is also related to U.S. patent applicationSer. No. ______, titled “Ballast For Fluorescent Emergency Lighting,”filed on Aug. 18, 2010, the entire contents of which are hereby fullyincorporated herein by reference.

TECHNICAL FIELD

The disclosure relates generally to apparatus and methods for providingemergency power to linear fluorescent lamps. More specifically, thedisclosure relates to apparatus and methods for providing current tofluorescent lamps using an emergency battery such that the life of thelamp is not shortened unnecessarily.

BACKGROUND

Most modern buildings have the capability to provide emergency lightingin the event of an interruption to the main power supply. Emergencylighting not only improves safety, but is required by most buildingcodes. Conventional emergency lighting systems work by identifying afailure in the main power supply and switching to a battery backup tosupply power to some of the lamps in the building until such time asmain power (sometimes referred to as mains power) is restored.

Although simple in concept, there are many issues that emergencylighting systems must resolve. One such issue relates to the fact thatmost buildings that have emergency lighting use linear fluorescent lampsto provide illumination. Fluorescent lamps are designed to run onalternating current. The batteries that provide emergency power providedirect current. Accordingly, the circuitry, or “ballast,” that providescurrent from the battery to the fluorescent lamp must convert thebattery's direct current into alternating current capable ofilluminating the fluorescent lamp.

Conventional emergency ballasts suffer from several limitations that canreduce their effectiveness. First, conventional emergency ballasts donot transform the battery's direct current into substantially sinusoidalalternating current. FIGS. 1 a, 1 b, and 1 c show the output waveformsof several conventional emergency ballasts. As the waveforms illustrate,none of these emergency ballasts output a substantially sinusoidalsignal. FIG. 1 a is effectively a triangle wave. FIG. 1 b has a strongdirect current bias in the negative region. FIG. 1 c similarly has astrong negative DC current bias. Although each of the signals shown inFIGS. 1 a, 1 b, and 1 c may be capable of illuminating a linearfluorescent lamp, the shape of the waveforms these emergency ballastsprovide can have serious deleterious effects on a linear fluorescentlamp.

These deleterious effects stem from the fact that wave patterns such asthose shown in FIGS. 1 a, 1 b, and 1 c accelerate the processes thatshorten the life of fluorescent lamps. Conventional fluorescent lampsrequire a certain amount of mercury vapor to be present within the lampto allow the lamp to start, or “strike.” Over the life of anyfluorescent lamp, the mercury begins to deposit itself at one end of thelamp. Eventually, as more and more mercury is deposited, the mercurylevel within the lamp falls to the point of no longer being sufficientto allow the lamp to strike. When the balance between the positive andnegative halves of the supply current are approximately equal—as theyare in a substantially sinusoidal wave—this process occurs at theslowest possible pace. When the signal has a strong direct-current bias,the process of mercury deposit accelerates.

A conventional way of determining whether the balance between positiveand negative current is appropriate is by graphing the current waveformand verifying symmetry with no DC bias. Conventionally, determining howsinusoidal a wave form is involves calculating the current crest factor.The current crest factor is half of the peak-to-peak value of the wave,divided by the root mean square (RMS) value for that wave. An idealcurrent crest factor (i.e., the current crest factor for a substantiallysinusoidal wave) is approximately 1.414. The ANSI (American NationalStandards Institute) standard current crest factor for fluorescent lampsis 1.7 or less. When the signal applied to the lamp has a strongerdirect current bias, such as the signals shown in FIGS. 1 a, 1 b, and 1c, it is also common for the current crest factor to exceed 1.7. Thelamps with a current crest factor above 1.7 will exhibit cathode erosionwhich will cause “sputtering” and shorten the lamp life. DC biasing willcause mercury to migrate toward one end of the lamp or the other. Thiscauses accelerated deposition of mercury on one end of the lamp, whichcan significantly shorten the life of a lamp. Referring again to FIGS. 1a, 1 b, and 1 c, the current crest factors of these waves areapproximately 2.02, 2.21, and 1.95, respectively, and accordingly, havecurrent crest factors that are too high for proper fluorescentoperation.

In practice, an emergency ballast that provides a current with a heavyDC bias to a linear fluorescent lamp can result in significantlyincreased expenses. For example, a linear fluorescent lamp that wouldnormally have a lifetime measured in years could fail in as little as,by way of example, 90 minutes of emergency operation. Accordingly, evena single power outage could result in the failure of conventional linearfluorescent lamps connected to the emergency ballast. Further, forenvironmental reasons, fluorescent lighting technology is moving towardthe use of less and less mercury in linear fluorescent lamps.Accordingly, with less mercury in the lamps to begin with, the problemis exacerbated, resulting in even shorter lamp life when under emergencyoperation.

A further problem with conventional emergency ballasts is that they donot provide cathode-heating features that are needed to maintain areasonable life cycle for reduced-mercury lamps. As one of skill in theart would recognize, fluorescent lamps operate by heating up a cathodeand causing it to emit electrons. The electrons ionize noble gas atomsin the lamp, causing the atoms to emit a photon that strikes thephosphors on the glass, causing light. To ionize properly, the gasseshave to be heated to a certain temperature, by way of example only,25-30 degrees centigrade for a standard T8 lamp. If a lamp were to becold-started, the initial voltage required to cause ionization would bevery high. Mercury is used in fluorescent lamps to allow the noblegasses to ionize at lower voltages.

Mercury, however, is highly toxic, and efforts are continuously made toreduce, if not eliminate, the mercury in fluorescent lamps. Becauseconventional emergency ballasts operate at a lower voltage, they may notprovide sufficient energy to strike a low-mercury lamp. Further, even ifa conventional emergency ballast did provide sufficient energy to strikea low-mercury lamp, the extra energy required can cause the cathode inthe lamp to age prematurely, unnecessarily reducing the life of thelamp.

SUMMARY

The present invention provides an emergency ballast that provides powerto a lamp that can minimize unnecessary aging of a lamp. In oneexemplary embodiment, an emergency ballast can receive direct currentfrom a battery in response to a loss of mains power. A conversioncircuit can convert the direct current into alternating current. Thealternating current is substantially sinusoidal, and can have a currentcrest factor of 1.7 or less. The conversion circuit can use a push-pulltopology to create the alternating current.

The ballast can also include a cathode-heating circuit that can pre-heatthe cathode of a lamp. The cathode-heating circuit can increase thecurrent output of the conversion circuit. The ballast can also includecircuitry for converting mains power to power that is appropriate forpowering the lamp.

The present invention also provides a method for providing emergencypower to a lamp. First, it is determined whether an interruption inmains power has occurred. In response to determining that aninterruption in mains power has occurred, direct current from a batteryis provided to a conversion circuit. The direct current is thenconverted to substantially sinusoidal alternating current, which canthen be provided to a lamp. The alternating current can have a currentcrest factor of 1.7 or less. The lamp can be a fluorescent lamp. Themethod can also heat a cathode of the lamp. The method can heat thecathode by increasing the current that is supplied to the cathode of thelamp. The method can also convert mains power to power that isappropriate for lighting the lamp in response to determining that mainspower has not been interrupted.

The present invention also provides a circuit for providing current to afluorescent lamp. The circuit can include a battery for deliveringdirect current in response to an interruption in mains power. Thecircuit can also include a conversion circuit for converting the directcurrent to alternating current. The conversion circuit can employ apush-pull topology. The alternating current can have a substantiallysinusoidal waveform with a current crest factor of 1.7 or less. Thealternating current can then be supplied to a fluorescent lamp. Thecircuit can also include a cathode heating circuit for pre-heating acathode of the lamp. The circuit can also include a ballast forconverting mains power to current appropriate for powering a fluorescentlamp.

These and other aspects, features, and embodiments of the invention willbecome apparent to a person of ordinary skill in the art uponconsideration of the following detailed description of illustratedembodiments exemplifying the best mode for carrying out the invention aspresently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the exemplary embodiments of thepresent invention and the advantages thereof, reference is now made tothe following description in conjunction with the accompanying drawingsin which:

FIG. 1 a is a waveform that demonstrates the output of a firstconventional emergency ballast;

FIG. 1 b is a waveform that demonstrates the output of a secondconventional emergency ballast;

FIG. 1 c is a waveform that demonstrates the output of a thirdconventional emergency ballast;

FIG. 2 is a flow chart describing an exemplary method for providingsubstantially sinusoidal output from an emergency fluorescent ballastaccording to one exemplary embodiment;

FIG. 3 is a circuit diagram illustrating the circuits in an exemplaryemergency ballast that provides substantially sinusoidal output from anemergency fluorescent ballast according to one exemplary embodiment;

FIG. 4 is a circuit diagram illustrating a magnified view of anexemplary circuit that implements the method of FIG. 2 for providingsubstantially sinusoidal output from an emergency fluorescent ballastaccording to one exemplary embodiment;

FIG. 5 is a waveform that demonstrates the output of the exemplarycircuit of FIG. 4;

FIG. 6 is a flow chart describing an exemplary method for providingcathode heating from an emergency fluorescent ballast according to oneexemplary embodiment;

FIG. 7 is a circuit diagram illustrating an exemplary circuit forproviding cathode heating according to one exemplary embodiment.

The drawings illustrate only exemplary embodiments of the invention andare therefore not to be considered limiting of its scope, as theinvention may admit to other equally effective embodiments. The elementsand features shown in the drawings are not necessarily to scale,emphasis instead being placed upon clearly illustrating the principlesof exemplary embodiments of the present invention. Furthermore,electrical components shown in the drawings and figures representexemplary circuits only. As one of skill in the art would understand,the electrical characteristics and response of certain components canoften be replicated by the use of other components and/or combinationsof components. In the drawings, reference numerals designate like orcorresponding, but not necessarily identical, elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention is directed to electrical lighting devices. Inparticular, certain exemplary embodiments of this invention are directedto providing substantially sinusoidal current to fluorescent lamps inthe event of a failure of main power. Certain exemplary embodimentsinclude an emergency ballast that can be powered by a battery or othersource of direct current, while still outputting substantiallysinusoidal current. In certain embodiments, the sinusoidal current has acurrent crest factor of 1.7 or less. In certain other exemplaryembodiments, the emergency ballast includes a cathode heating circuitthat improves the emergency ballast's ability to illuminate alow-mercury fluorescent lamp.

The invention may be better understood by reading the followingdescription of non-limiting, exemplary embodiments with reference to theattached drawings, wherein like or corresponding, but not necessarilyidentical, parts of each of the figures are identified by the samereference characters, and which are briefly described as follows. FIG. 2is a flow chart describing an exemplary method 200 for providing asinusoidal output from an emergency fluorescent ballast. In step 205, afluorescent lamp is operated with the fluorescent AC ballast, whichconverts mains power to power that is appropriate for illuminating afluorescent lamp.

In step 210, it is determined whether an interruption in AC power hasoccurred. In one exemplary embodiment, this determination is made by therelays in the system, which receive AC current across their windings,and switch from the Normally Closed position to the Normally Openposition when power is cut off. If the determination in step 210 isnegative, the NO branch is followed back to step 205, wherein operationof the lamp continues from the AC fluorescent ballast. If, however, thedetermination in step 210 is affirmative, the method 200 follows the YESbranch to step 215, wherein power to the lamp is switched from the ACfluorescent ballast to the emergency ballast. The method 200 thenproceeds to step 220, wherein the emergency ballast powers the lamp byconverting DC power from a DC power source, such as a battery, to asubstantially sinusoidal AC signal having a current crest factor of 1.7or less.

The method 200 then proceeds to step 225, wherein it is determinedwhether AC power has been restored. If the determination in step 225 isaffirmative, the method 200 follows the YES branch to step 205. If, onthe other hand, the determination in step 225 is negative, the method200 follows the NO branch to step 220.

FIG. 3 is an exemplary circuit diagram for an emergency ballast 300 thatoutputs a substantially sinusoidal AC signal having a current crestfactor of 1.7 or less according to one exemplary embodiment. Theemergency ballast 300 includes a battery 302. In an exemplaryembodiment, the battery 302 is a nickel-cadmium rechargeable battery.Alternatively, the battery 302 is a nickel-metal hydride, lithium-ion,or any other rechargeable battery. In yet another alternativeembodiment, the battery 302 is not rechargeable. The emergency ballast300 also includes several inputs and outputs 304, 306, 308, 310, 312,314. The emergency ballast 300 has an input 304 that is electricallycoupled to the main AC power supply from the building (the “hot” wire,or “mains power”). The emergency ballast 300 also has inputs 306 and 310that are electrically coupled to the lamp to be powered (not shown). Theemergency ballast 300 also has inputs 308 and 312 that are electricallycoupled to the AC fluorescent ballast. The emergency ballast 300 alsohas an output 314 that is electrically coupled to the lamp to bepowered. The emergency ballast 300 also includes a circuit 400 thatconverts the DC output of the battery 302 to sinusoidal AC output. Thecircuit 400 will be discussed in further detail with respect to FIG. 4.

Turning now to FIG. 4, a circuit diagram illustrating a magnified viewof an exemplary circuit 400 for providing a sinusoidal output from anemergency fluorescent ballast according to one exemplary embodiment ispresented. Generally, the circuit 400 is electrically coupled to thepositive and negative terminals of the battery 302 (FIG. 3), andincludes components to invert the direct current received from thebattery 302 (FIG. 3) into alternating current having a substantiallysinusoidal output with a current crest factor of 1.7 or less. In theexemplary circuit 400 shown in FIG. 4, the circuit 400 employs discretecomponents and a push-pull topology to convert the battery 302 (FIG. 3)current into substantially sinusoidal alternating current. Inalternative exemplary embodiments, however, the circuit 400 employs ahalf-bridge topology. In another alternative exemplary embodiment,rather than using discrete components, the circuit employs an integratedcircuit configured to perform the same function.

The exemplary circuit 400 includes an input 402 that is electricallycoupled to and receives the positive output of the battery 302 (FIG. 3).The exemplary circuit 400 also includes input 404 that is electricallycoupled to and receives current from the negative terminal of thebattery 302 (FIG. 3). The positive output of the battery is passedthrough an inductor 406 that acts as a “choke” to limit the currentbeing supplied to the transistors and remove DC bias. The current isthen passed to the center tap of a transformer 408.

To create the substantially sinusoidal AC output, the negative terminalof the battery is alternatively electrically coupled to the firstterminal 410 of the transformer 408, and then the other terminal 412 ofthe transformer 408. By repeatedly switching the terminal 410, 412 ofthe transformer 408 that is electrically coupled to the negativeterminal of the battery 302 (FIG. 3), an alternating current is inducedin the transformer 408.

The terminal switching is controlled by transistors 414 and 416 inconjunction with resistors 420, 422, and 424, and inductor 418.Resistors 420, 422, and 422, are tuned with inductor 418 to switchtransistors 414 and 416 on and off such that the resulting output fromtransformer 408 is a substantially sinusoidal AC current with a currentcrest factor of 1.7 or less. By way of example only, the inductor 406 isa feed choke having an E1187 core, an inductance of 0.095 mH, and an RDCof 0.045 ohms. Transformer 408 has a primary winding with 7 turns,inductance of 0.004 mH, and RDC of 0.05 ohms. The secondary winding oftransformer 408 has 1000 turns, inductance of 163 mH, and resistance of44 ohms. Transistor 418 has 2 turns, an inductance of 0.00065 mH, and aresistance of 0.03 ohms. Resistor 424 is a ½ watt, 150 ohm resistor.Resistors 418 and 420 are ¼ watt, 1 ohm resistors. Capacitor 428 is a600 volt AC, 330 pF film capacitor. As one of skill in the art wouldrecognize, however, other combinations of resistors, inductors, andother components would result in a sinusoidal output from the emergencyballast.

As a result of the switching, transformer 408 produces a sinusoidal ACoutput at its output terminals 426 and 430. A first terminal 426 iselectrically coupled to the AC fluorescent ballast. A second terminal430 is electrically coupled to the cathode side of the lamp. In theexemplary embodiment, a capacitor 428 can be coupled to the cathodeterminal to further smooth the output waveform.

Turning now to FIG. 5, a waveform demonstrating the output of thecircuit 400 of FIG. 4 is shown. As shown in FIG. 5, the output has acurrent crest factor of 1.38, which is very close to the ideal currentcrest factor of 1.414. The current crest factor shown in FIG. 5 is alsowell under the ANSI recommended maximum current crest factor of 1.7.Because the output of FIG. 5 has a current crest factor so close to theideal, a fluorescent lamp coupled to the emergency ballast will notexperience unnecessary ageing, due to high current crest factor (cathodesputtering) or DC bias (mercury migration) as would likely occur withany of the emergency ballast outputs shown in FIGS. 1 a, 1 b, and 1 c.

Turning now to FIG. 6, a flow chart describing an exemplary method 600for providing cathode heating from an emergency fluorescent ballastaccording to one exemplary embodiment is provided. In step 605, currentis received from an emergency ballast. In one exemplary embodiment, thecurrent in step 605 is the current output of an emergency ballast suchas emergency ballast 300 of FIG. 3. In step 610, it is determinedwhether cathode heating will be used. If the decision in step 610 isaffirmative, the method 600 follows the “Yes” branch and proceeds tostep 615, in which the level of current is increased. In an exemplaryembodiment, the current is increased according to the specification of alamp. By way of example only, for a T8 lamp, the current is increased by350-400 milliamps. The method 600 then proceeds to step 620, wherein theincreased current is supplied to the lamp.

Turning again to step 610, if the determination is negative, the method600 follows the NO branch and proceeds to step 620, wherein anon-amplified current is supplied to the lamp. The method 600 thenproceeds to step 625, wherein it is determined whether AC power has beenrestored. If the decision in step 625 is negative, the NO branch isfollowed and the method 600 returns to step 605. If the decision in step625 is affirmative, the YES branch is followed and the method 600 ends.

Turning now to FIG. 7, a circuit diagram illustrating an exemplarycircuit 700 for providing cathode heating according to one exemplaryembodiment is shown. As shown, the exemplary circuit 700 is provided asa modification of the substantially sinusoidal AC output circuit 400provided in FIG. 4. As one of ordinary skill in the art wouldunderstand, however, the circuit 700 for cathode heating can be appliedto any implementation of a fluorescent ballast, including other existingemergency ballast implementations that do not provide sinusoidal ACoutput, such as those referenced in FIGS. 1 a, 1 b, and 1 c. To theextent that the exemplary embodiment of a cathode heating circuit 700modifies the circuit 400 described in FIG. 4, components alreadydescribed in FIG. 4 will be described using the same reference numeralsused in FIG. 4.

In an exemplary embodiment, the outputs 426 and 430 of the circuit 400are electrically coupled to windings 706 and 708. The exemplary windings706 and 708 are inductively coupled to the transformer 408 and are tunedto drive an increased current to the lamp. In one exemplary embodiment,the output of the windings 706 and 708 is passed through smoothingcapacitors 710 and 712 to remove unwanted noise from the signal.Capacitors 710 and 712 adjust the filament current, and are used tofine-tune the cathode current output from the windings 706 and 708. Byway of example only, windings 706 and 708 have an inductance of 0.01 mHand 8 turns. Capacitors 710 and 712 are 63 volt, 220 nF film capacitors.

The windings 706 and 708 increase the current applied to the lampwithout requiring a corresponding increase in the output of the powersource, for example, the battery 302 (FIG. 3). The increased currentheats the cathodes, thus reducing the amount of voltage necessary tosuccessfully strike the lamp, which allows the emergency ballast 300 tooperate low mercury fluorescent lamps. Although the exemplary embodimentprovides additional windings 706 and 708 to increase the current outputfrom the emergency ballast, one of skill in the art would understandthat other combinations of components could achieve the same result.

The emergency ballast 300 providing substantially sinusoidal output canalso be combined with a standard AC fluorescent ballast to provide acomplete fluorescent ballast solution. When combined with a standardfluorescent ballast, the emergency ballast 300 may further be modifiedwith the cathode heating circuit 700.

Although the invention has been described with reference to specificembodiments, these descriptions are not meant to be construed in alimiting sense. Various modifications of the disclosed embodiments, aswell as alternative embodiments of the invention will become apparent topersons of ordinary skill in the art upon reference to the descriptionof the invention. It should be appreciated by those of ordinary skill inthe art that the conception and the specific embodiments disclosed maybe readily utilized as a basis for modifying or designing otherstructures or methods for carrying out the same purposes of theinvention. It should also be realized by those of ordinary skill in theart that such equivalent constructions do not depart from the spirit andscope of the invention as set forth in the appended claims. It istherefore, contemplated that the claims will cover any suchmodifications or embodiments that fall within the scope of theinvention.

1. A ballast for a luminaire, comprising: a battery that provides directcurrent in response to a loss of mains power to the luminaire; aconversion circuit, wherein the circuit receives the direct current fromthe battery, converts the direct current into substantially sinusoidalalternating current, and provides the alternating current to a lamp inthe luminaire.
 2. The ballast of claim 1, wherein the substantiallysinusoidal alternating current has a current crest factor of 1.7 orless.
 3. The ballast of claim 1, wherein the loss of power comprises aloss of a mains power.
 4. The ballast of claim 1, wherein the conversioncircuit comprises a push-pull topology.
 5. The ballast of claim 1,further comprising a cathode-heating circuit configured to pre-heat acathode of the lamp.
 6. The ballast of claim 6, wherein thecathode-heating circuit increases the current output of the conversioncircuit.
 7. The ballast of claim 1, wherein the lamp is a fluorescentlamp.
 8. The ballast of claim 1, further comprising circuitry forconverting mains power to power appropriate to illuminate the lamp.
 9. Amethod for providing emergency power to a lamp, comprising: determiningwhether mains power has been interrupted; providing direct current to aconversion circuit in response to a determination that mains power hasbeen interrupted; converting the direct current to alternating current,wherein the alternating current is substantially sinusoidal; andproviding the alternating current to the lamp.
 10. The method of claim9, wherein the alternating current comprises a current crest factor of1.7 or less.
 11. The method of claim 9, wherein the lamp comprises afluorescent lamp.
 12. The method of claim 9, wherein the conversioncircuit comprises a push-pull topology.
 13. The method of claim 9,wherein the direct current is supplied by a battery electrically coupledto the conversion circuit.
 14. The method of claim 9, further comprisingthe step of heating a cathode of the lamp.
 15. The method of claim 14,wherein heating the cathode comprises increasing the current to acathode of the lamp.
 16. The method of claim 9, further comprisingconverting mains power to power appropriate for lighting the lamp inresponse to determining that mains power has not been interrupted.
 17. Acircuit for providing current to a fluorescent lamp, comprising: abattery configured to deliver direct current in response to aninterruption in mains power; and a conversion circuit configured toconvert the direct current to alternating current, wherein thealternating current comprises a substantially sinusoidal waveform havinga current crest factor of 1.7 or less, and to provide the alternatingcurrent to a fluorescent lamp.
 18. The circuit of claim 16, wherein theconversion circuit comprises a push-pull topology.
 19. The circuit ofclaim 16, further comprising a cathode heating circuit for pre-heating acathode of the lamp.
 20. The circuit of claim 16, further comprising aballast for converting mains power to current appropriate for powering afluorescent lamp.